1. Field of the Invention
The present invention relates to a lithographic apparatus and a method for aligning portions of a lithographic apparatus.
2. Background
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.
Lithography is widely recognized as one of the key steps in the manufacture of ICs and other devices and/or structures. However, as the dimensions of features made using lithography become smaller, lithography is becoming a more critical factor for enabling miniature IC or other devices and/or structures to be manufactured.
A theoretical estimate of the limits of pattern printing can be given by the Rayleigh criterion for resolution as shown in equation (1):
                    CD        =                              k            1                    *                      λ            NA                                              (        1        )            where λ is the wavelength of the radiation used, NA is the numerical aperture of the projection system used to print the pattern, k1 is a process dependent adjustment factor, also called the Rayleigh constant, and CD is the feature size (or critical dimension) of the printed feature. It follows from equation (1) that reduction of the minimum printable size of features can be obtained in three ways: by shortening the exposure wavelength λ, by increasing the numerical aperture NA or by decreasing the value of k1.
In order to shorten the exposure wavelength and, thus, reduce the minimum printable size, it has been proposed to use an extreme ultraviolet (EUV) radiation source. EUV radiation is electromagnetic radiation having a wavelength within the range of 5-20 nm, for example within the range of 13-14 nm, or example within the range of 5-10 nm such as 6.7 nm or 6.8 nm. Possible sources include, for example, laser-produced plasma sources, discharge plasma sources, or sources based on synchrotron radiation provided by an electron storage ring.
EUV radiation may be produced using a plasma. A radiation system configured to produce EUV radiation may include a laser configured to excite a fuel to provide the plasma, and a source collector module configured to contain the plasma. The plasma may be created, for example, by directing a laser beam at a fuel, such as particles of a suitable material (e.g., tin), or a stream of a suitable gas or vapor, such as Xe gas or Li vapor. The resulting plasma emits output radiation, e.g., EUV radiation, which is collected using a radiation collector. The radiation collector may be a mirrored normal incidence radiation collector, which receives the radiation and focuses the radiation into a beam. The source collector module may include an enclosing structure or chamber arranged to provide a vacuum environment to support the plasma. Such a radiation system is typically termed a laser produced plasma (LPP) source.
The orientation and/or position of the collector will determine the direction in which radiation is directed from the collector (e.g., reflected from the collector). It is desirable that radiation collected by the collector is accurately directed to parts of the lithographic apparatus, and it is therefore desirable for the collector to direct radiation in a specific direction. The radiation directed in a specific direction may be referred to as a radiation beam. An illuminator (sometimes referred to as an “illumination system” or “illumination arrangement”) is a part of the lithographic apparatus that receives a radiation beam from the collector. The illuminator may condition the radiation beam and direct it to a patterning device.
The projection of the radiation beam onto a particular plane may be referred to as the far field. Far field referred herein may thus be understood as the projection of the radiation beam that enters the illumination system onto a particular plane situated in the path of the radiation beam. Hence the far field is an image (onto the particular plane) of the radiation beam directed at the particular plane by the collector. The projection of the radiation beam on to any appropriate plane may be referred to as the far field. An example of a plane on to which the radiation beam may be projected (and thus form the far field) is the first optical surface of the illuminator. The first optical surface of the illuminator may be a surface of a reflector. The reflector may be the first reflector the radiation beam is incident upon as it traverses the illuminator. The far field position is a reference position which may be used to describe the location of the far field. The far field position may be any appropriate position which describes the location of the far field. For example, the far field position may be the centre center of the image formed on the plane of the far field (i.e., the center of the projection). The center of the image may be the geometric center of the image, or in some cases the center of the image may be the position of the average center of the power distribution of the image.
The far field (and hence the far field position) are preferably located within certain boundaries in order to improve the operating performance of the illuminator (and hence the imaging performance and/or throughput of the lithographic apparatus). For example, if the far field is the projection of the radiation beam on to the surface of the first reflector the radiation beam is incident upon as it traverses the illuminator, it may be desirable for the far field to be such that it substantially does not extend beyond the extent of the surface of the first reflector. Alternatively or in addition, it may be desirable for the far field position (which in this case is defined by the geometric center of the projection) to be located substantially at the geometric center of the surface of the first reflector. For example, if the surface of the reflector is substantially circular and the far field projected on to the surface of the reflector is also substantially circular, then the radius of the far field may be substantially equal to the radius of the surface of the reflector and the far field position may be located substantially at the geometric center of the reflector.
The location of the far field (and hence the far field position) depends on the collector orientation and position. When a lithographic apparatus is constructed and used for the first time, it may be possible to ensure that the collector directs radiation in such a specific direction. However, over time it can be difficult to ensure that the radiation beam is directed in this specific direction. For instance, movement of parts of the lithographic apparatus (e.g., parts of the radiation source) can shift the direction of radiation. Additionally or alternatively, when parts of the lithographic apparatus are replaced (e.g., for maintenance purposes) a misalignment of replacement parts can shift the direction of radiation.
It is therefore desirable to align or re-align a collector of a radiation source and parts of the lithographic apparatus located further along the path of the radiation beam. Because the illuminator is a part of the lithographic apparatus that receives radiation directed by the collector it may be desirable to align or re-align the collector and the illuminator so that the far field (and hence far field position) is located within certain boundaries.
According to a first aspect of the invention there is provided a lithographic apparatus comprising a source collector module including a collector, configured to collect radiation from a radiation source; an illuminator configured to condition the radiation collected by the collector and to provide a radiation beam; and a detector arrangement comprising a reflector arrangement disposed in a fixed positional relationship with respect to the illuminator, the reflector arrangement being arranged to reflect radiation from the source collector module; and a sensor arrangement disposed in a fixed positional relationship with respect to the reflector arrangement, the sensor arrangement being configured to measure at least one property of radiation reflected by the reflector, the detector arrangement being configured to determine the location of a far field position of the radiation as a function of at least one property of the radiation reflected by the reflector and measured by the sensor arrangement.
The far field position may be the geometric center of the far field or the average center of the power distribution of the far field.
The sensor arrangement may comprise a one-dimensional sensor.
The reflector arrangement may comprise a generally planar reflector.
The reflector arrangement may further comprise an isolated reflector region.
The reflector arrangement may further comprise a substantially unreflective region and a reflective region, the substantially unreflective region being between the reflective region and the isolated reflector region.
The substantially unreflective region, reflective region and isolated reflector region may be formed as a one-piece component or as integral components.
The reflector arrangement may comprise a curved reflector.
The radius of curvature (R) of the reflector may be given by
  R  =            4      ⁢      ab              (              b        +                  2          ⁢          a                    )      
where a is the distance between an intermediate focus of the radiation collected by the collector and a center of the curved reflector and b is the distance between the center of the curved reflector and the sensor arrangement.
The curvature of the reflector may be configured such that a change in far field position measured by the detector arrangement due to a relative translation between the source collector module and illuminator by a distance of 1 mm is the same as a change in far field position measured by the detector arrangement due to a relative tilt between the source collector module and illuminator about an intermediate focus of 1/a mrad, where a is the distance between an intermediate focus of the radiation collected by the collector and a center the curved reflector.
The curvature of the reflector may be configured such that a change in far field position measured by the detector arrangement due to a relative translation between the source collector module and illuminator by a distance of 1 mm is the same as a change in far field position measured by the detector arrangement due to a relative tilt between the source collector module and illuminator about an intermediate focus of 1/a mrad, where a is the distance between an intermediate focus of the radiation collected by the collector and a center of the curved reflector. The lithographic apparatus may additionally comprise an isolated reflective feature, at least part of which is mounted to or forms part of the collector, wherein the isolated reflective feature comprises a reflector portion located in a fixed positional relationship with the collector at a radial distance which is less than the radius of the collector, the reflector portion being surrounded by a relatively unreflective portion.
The sensor arrangement may comprise a two-dimensional sensor.
The two-dimensional sensor may be a position sensitive device (PSD) or a charge coupled device (CCD).
The one-dimensional sensor may be an edge detection sensor.
The detector arrangement may comprise a plurality of similar reflector arrangements and corresponding similar sensor arrangements.
According to a second aspect of the invention there is provided a method of aligning a source module and an illuminator of a lithographic apparatus, the source module including a collector configured to collect radiation from a radiation emitting plasma; the illuminator configured to condition the radiation collected by the collector and to provide a radiation beam; the lithographic apparatus further comprising:
an actuator; and a detector arrangement comprising a reflector arrangement disposed in a fixed positional relationship with respect to the illuminator; and a sensor arrangement disposed in a fixed positional relationship with respect to the reflector arrangement; the method comprising the sensor arrangement measuring at least one property of radiation reflected by the reflector, the detector arrangement determining the location of a far field position of the radiation as a function of at least one property of the radiation reflected by the reflector and measured by the sensor arrangement; comparing the location of the far field position measured by the detector arrangement to a target far field position; and the actuator producing a relative movement between at least part of the source collector module and at least part of the illuminator to move the location of far field position toward the target far field position.